JP3756744B2 - Forging die damage morphology prediction method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for predicting the form of damage occurring in a forging die, and further predicting the life of the forging die.
[0002]
[Prior art]
In forging, a large force and high heat are applied to the forging die, and the forging die is easily damaged and its life is short. For this purpose, the forging process is devised to adopt a forging process that extends the die life as much as possible. However, at present, a technique for predicting the forging die life to a reliable level has not been developed, and the forging process is determined by trial and error.
In the world, technologies leading to forging die life prediction technology have been developed in pieces. For example, a conditional expression for predicting softening of steel accompanying tempering heating has been proposed (Iron and Steel, No. 10, 1980, Satoshi Inoue). However, this technology is limited to the temper softening prediction of steel for machine structural use, and the damage form and life cannot be predicted. Although the failure distribution form has been studied (Plastic Processing Spring Lecture Collection Vol. 1, 1996, Shinichiro Fujikawa), the damage form cannot be specified, so the damage form cannot be predicted. Research on the fracture theory of materials is also progressing (The 45th Plastic Processing Lecture 29, 1994, Miyahara et al.), But it is limited to fatigue fracture and is not sufficient for predicting the damage form and life expectancy of forging dies. .
In Japanese Patent Laid-Open No. 10-175037, the finite element method is applied to a forging die to calculate the plastic deformation stress and the maximum principal stress acting on the die, and the crack progresses from the calculated plastic deformation stress amplitude and the maximum principal stress amplitude. This shows a technique for calculating the service life by calculating the speed and calculating the number of machining times until the calculated crack depth reaches a predetermined depth. However, the life of a die is not only determined by the progress of cracks, but rather in the case of a hot forging die, wear often progresses to the die life. A technique for predicting the life by calculating the crack depth can calculate a reliable life only in a very limited case.
[0003]
When there are multiple forging process proposals that can produce the same forging results, calculate the deformation process of the material and the hydrostatic pressure and load applied to the forging die, and determine the calculation results by the person and see the die life The forging process that seems to be used is adopted. However, in practice, knowledge about what is optimal is not obtained, and an optimal forging process is not necessarily selected. It remains at the trial and error stage.
[0004]
[Problems to be solved by the invention]
Therefore, in the present invention, a technique capable of predicting the form of damage occurring in the forging die is developed. Furthermore, a technology for predicting the life of the forging die from the damage form of the forging die will be developed. The form of damage that occurs in the forging die is closely related to the die life, and if the damage form can be predicted, the validity of the forging process can be objectively evaluated. Alternatively, it is possible to evaluate whether or not the shape of the forged product is unreasonable.
[0005]
[Means, Actions and Effects for Solving the Problems] In the present invention, attention is paid to the fact that the load that acts on the forging die to determine the life is roughly divided into a mechanical load and a thermal load. The mechanical load can be regarded as an index for deforming or wearing the forging die as represented by the stress acting on the die. As the mechanical load, cumulative friction work, surface area expansion ratio, sliding distance, sliding speed, and the like can be adopted. The thermal load can be said to be an index of easiness when the forging die is deformed or worn, as represented by the fact that the forging die is tempered by repeatedly applying a high temperature to the forging die. A yield strength ratio or the like can be adopted as the thermal load.
The present invention is based on finding and confirming that a clear relationship can be obtained as a result of analyzing the relationship of “mechanical load—thermal load—damage form occurring in the forging die”.
The form of damage that occurs in the forging die is classified into “heat check, heat crack, streak, deformation, small wear, large wear” and the like. Here, if the damage forms that occur in the forging die under the combined state of the loads based on the mechanical load and the thermal load acting on the forging die are classified, the combined state of the load and the damage form correspond well. It was confirmed.
Therefore, once a database showing the relationship of "mechanical load-thermal load-damage form generated in the forging die" is constructed, the subsequent steps are based on the mechanical load and thermal load acting on the forging die. It is possible to predict well the form of damage that occurs in the combined state of the load.
[0006]
In the first aspect of the present invention, a database is constructed in which a mechanical load and a thermal load acting on the forging die are input and a damage form generated in the forging die is output in a combined state of the loads. Then, the mechanical load and the thermal load acting on the forging die being planned are calculated. The damage combination generated in the forging die is predicted by inputting the calculated combination of loads into the database and outputting the damage form from the database.
[0007]
According to this method, it is possible to accurately predict the form of damage that occurs in the forging die under the combined state of the load from the “mechanical load and thermal load acting on the forging die” that can be calculated. it can. Knowing the form of damage is extremely effective during the design or evaluation of the forging process, for example, it can be seen that there is no difficulty in the forging process when forging is completed while heat check, heat cracking occurs, On the other hand, it is immediately understood that a review of the forging process is required when large wear forms occur.
[0008]
It is particularly useful to take the cumulative work of friction as the mechanical load and the yield strength ratio as the thermal load. In this case, the damage forms are clearly classified in a two-dimensional plane in which the cumulative frictional work is taken on one axis and the yield strength ratio is taken on the axis perpendicular to it, and read from the cumulative frictional work and yield strength ratio. Damage form is very accurate and error free.
[0009]
The forging die used in the past, the combination of mechanical load and thermal load, it is possible to create the forging die before Symbol database to investigate the failure mode that occurs at that time. In this way, it is not necessary to conduct separate experiments or tests, and a highly reliable database suitable for the actual situation is constructed.
[0010]
In the second aspect of the invention, the lifetime is predicted by paying attention to the form of damage. In this aspect, the mechanical load and the thermal load acting on the forging die being planned are calculated for each forging process. Then, a combination of loads calculated for each forging process is input to the database, and the number of forging processes until the damage form output from the database becomes a large wear form is calculated.
[0011]
The number of times calculated in this way is the number of times (corresponding to the minimum life) indicating that forging is possible at least the number of times of processing.
Through our research, it has been found that the forging die begins to wear out greatly when it is tempered and softened, and a large mechanical load is applied thereto, and then reaches the end of its life. Therefore, when a combination of loads calculated for each number of forging processes is input to the database, the damage mode output from the database can be forged without any problems other than the large wear mode, and the number of processing times is When it increases, it starts to output a large form of wear from the database, and it turns out that the life is near.
In the present invention, the lifetime is calculated using this knowledge. The life calculated here has a margin, and if the forging process is corrected using this as an index, a forging process satisfying the given life can be obtained.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The main features of the embodiments described below are listed first.
(Mode 1) As the mechanical load, any one of a surface area expansion ratio, a sliding distance, and a sliding speed is adopted.
(Form 2) The yield strength of the die used for calculating the yield strength ratio adopted as the thermal load is the value of λ calculated from the activation energy, temperature and duration required for the softening reaction, and the number of forging operations. It is calculated from the hardness information obtained by accumulating the effects of the high temperature and duration applied each time.
(Embodiment 3) The yield strength is calculated from the high-temperature deformation resistance of various mold materials that have been experimentally determined in advance and the temperature of the forging die during forging.
(Mode 4) The method according to claim 4 is applied to a forging die currently in use to predict the life. The method of the present invention is most effective when used in the forging die preparation stage, but can also be used to predict the remaining life by applying it to the forging die currently used.
[0013]
【Example】
FIG. 1 shows the overall processing procedure of this embodiment. In step S1, the shape of the forged product is designed. In step S2, a forging process is designed. In a normal forging process, the material is not forged at a time and processed into a final shape, but is forged into a plurality of times and processed into a final shape as illustrated in FIG. For the sake of simplicity, FIG. 5 shows a state where the final shape is processed by two forgings, but in practice, the final shape may be processed by many forgings. The forging process here refers to a chain of what shape is processed in each forging process to reach the final shape.
[0014]
In step S6 of FIG. 1, geometric data such as a material prototype, an intermediate shape, and a final shape are created and input to the computer for numerical calculation by the computer. In step S8, various data necessary for executing the heat-deformation coupled analysis by the computer are input to the computer. For this purpose, press D / B (hereinafter D / B indicates a database) D1, workpiece deformation resistance D / B (D2), interface characteristic D / B (D3), thermophysical property value D / B (D4) ) Is used.
[0015]
In step S10, using the data input in steps S6 and S8, a heat-deformation coupled analysis is performed using an electronic computer. Here, the finite element method is basically executed. This detail has already been published in the paper by the present inventors. For example, it has been published in Akashi et al .: 1998 Spring Plasticity Lecture (1998), 325-326, or Yano et al .: 49th Plasticity Lecture (1998), 91-92.
As a result of this analysis, it is analyzed what stress acts on what part of the material, what temperature, what kind of deformation. FIG. 6 illustrates an example of the analysis result, and analyzes the case where the material shown on the left is forged into the right shape.
[0016]
Steps S12 and after in FIG. 1 are the parts improved this time. In step S12, the monitoring position on the mold surface is designated. In normal cases, the position of the forging die is designated because it is known from the shape of the forging die and the past experience that the point at which the die life ends due to the most severe wear. If the point cannot be narrowed down to one location, the possible points are specified in order. If it is completely unknown, the monitoring position may be changed mechanically and regularly so as to cover all points of the forging die.
[0017]
In step S14, a mechanical load acting on the monitoring position, here, a cumulative friction work is calculated. The friction work here is the friction force (μp, where μ is the friction coefficient, p is the surface pressure) acting between the material and the forging die, multiplied by the slip distance, and this is forged once. What is integrated over the period is the cumulative friction work. The cumulative friction work W is represented by ∫μpvdt. Here, v is a relative sliding speed. FIG. 7 exemplifies a cumulative friction work amount applied to various places (9 places in total) of the punch in the finishing process of FIG. The horizontal axis shows the elapsed time during one forging process, and the cumulative friction work increases as the molding time elapses. The value at the end of machining is the cumulative work of friction here.
[0018]
In step S16 and subsequent steps in FIG. 1, the thermal load acting on the forging die is calculated. In step S16, the temperature of the forging die is calculated. Here, hot forging is employed, and the workpiece is originally heated. Further, the forging die is heated to a high temperature in order to generate heat due to friction accompanying forging. FIG. 8 shows a temperature change pattern generated everywhere in the punch during one processing. The left side of FIG. 9 shows how the forging die is heated as the number of machining operations increases, and one peak corresponds to one forging process (hereinafter sometimes referred to as a shot). The forging die is heated by forging, and after forging is finished, the lubricant is sprayed and cooled. The mold temperature gradually rises as this cycle is repeated, and eventually heating and cooling are repeated within the same temperature range.
[0019]
In step S18 of FIG. 1, the yield strength of the forging die softened by heat is calculated. At this time, D / B (D5) of the mold material λ value is used.
λ is a value serving as an index of ease of softening, and is calculated by the equation (92) in FIG. Here, Q is the active energy required for the softening reaction specific to the material, and is measured for each steel type. T is the absolute temperature of the working temperature and t is the duration. The value of λ increases as it is exposed to high temperatures for a long time.
When the temperature to be applied is not uniform, the change in the value of λ can be calculated by dividing every short time when the temperature can be regarded as uniform and adopting the formula (90). As a result, as shown on the right side of FIG. 9, the value of λ increases with time.
The value of λ determines the hardness of the forging die. The hardness decreases as the value of λ increases. The graph of Hv on the right side of FIG. 9 shows this, and shows the change in the hardness of the forging die when exposed to the temperature change shown on the left side of FIG.
The degree of softening that the heat imparts to the mold also varies depending on the stress acting on the mold. Therefore, it is more accurate to correct the expressions (90) and (92) by applying the influence of stress.
The hardness Hv shown on the right in FIG. 9 is the hardness at room temperature. FIG. 10 shows the relationship between temperature and hardness or yield strength, and the yield strength decreases at higher temperatures. Therefore, in actuality, the hardness at room temperature is obtained from the relationship on the right side of FIG. 9, and this curve is identified as the hardness at room temperature in FIG. 10, and machining is performed from the identified curve and the processing temperature. Calculate the yield strength of the hour. Thus, the yield strength at the processing temperature of the forging die heated and softened is calculated. Further, the yield shear stress of the forging die is calculated from this yield strength σ _H.
[0020]
In step S20 in FIG. 1, the frictional shear stress μp acting on the monitoring position where the yield strength is calculated is calculated. As described above, μ is a coefficient of friction and p is a surface pressure.
In step S22, the yield strength ratio is calculated as the thermal load in this case. Here, the yield strength ratio is a value obtained by dividing the yield strength σ _H by the frictional shear stress μp.
[0021]
The above processing can be applied to all forging dies whose shape and processing conditions are known. Therefore, by investigating a forging die that has been used in the past and has reached the end of its life, it is possible to investigate the relationship between the cumulative amount of friction work that has acted on the die and the yield strength ratio at the end of its life.
The cumulative friction work calculated in step S14 is substantially constant throughout the life of the forging die. On the other hand, the yield strength ratio calculated in step S22 decreases as the number of shots increases as the number of shots increases as shown in FIG. 9, so that the yield strength ratio also decreases as the number of shots increases. FIG. 11 shows this tendency. Since the number of shots at the end of the lifetime is known, the yield strength ratio at the end of the lifetime is calculated. In addition, by examining the forging die, it is possible to examine the form of damage that has occurred when the life is exhausted.
A similar investigation is possible for a forging die in use, and the relationship between the cumulative friction work-yield strength ratio-damage form at the time of the investigation can be examined.
Collect many relations of "cumulative friction work-yield strength ratio-damage form" for the molds that are currently in use or have reached the end of their lives, and show the cumulative friction work on the horizontal axis. FIG. 3 shows the damage form plotted on a two-dimensional plane with the yield strength ratio on the vertical axis.
[0022]
Obviously, a large wear damage pattern was observed in the lower right region, and they were out of service life.
The database of “mechanical load (cumulative frictional work) −thermal load (yield strength ratio) −damage form (damaged area)” is divided into five kinds of damage forms as shown in FIG. 2 in a simplified manner. being classified. Here, the characteristic of each damage form is shown in FIG. Here, large wear and small wear are classified according to the degree of wear, and can be clearly distinguished with the naked eye from FIG.
This two-dimensional map is clear. When the damage form is plotted on a two-dimensional plane with the mechanical load (cumulative frictional work) on one axis and the thermal load (yield strength ratio) on the other axis, It was confirmed that the form was clearly divided by range.
[0023]
Since this regularity was confirmed, the relationship of “mechanical load (cumulative friction work) −thermal load (yield strength ratio) −damage form” does not hold only for a specific forging die but is general. It was confirmed that
[0024]
Therefore, even when analyzing the planned forging die for the designed forged product, the mechanical load (cumulative friction work) calculated in step S14 in step S24 of FIG. 1 and in step S22. By searching the database of “mechanical load (cumulative frictional work) −thermal load (yield strength ratio) −damage form” shown in FIG. 3 using the calculated thermal load (yield strength ratio) as a key. The form of damage can be identified.
[0025]
As described above, the mechanical load (cumulative friction work) does not change greatly with the number of shots. On the other hand, the thermal load (yield strength ratio) changes from large to small with the number of shots.
Therefore, it usually changes as indicated by an arrow on the map of FIG. Usually, what was not in a large wear form initially shifts to a large wear area as the number of shots increases.
While the number of shots is small, NO is obtained in step S26 of FIG. In this case, the damage form at that time is displayed (step S28), the number of shots is increased by 1 (step S30), and step S18 and subsequent steps are repeated to calculate the yield strength ratio (thermal load) of the next shot. Among them, step S26 becomes YES when the damage form changes and the wear form becomes large. Here, the number of times of processing (number of shots) until the damage form becomes a large wear form is compared with a predetermined value, and if it is equal to or greater than the target number, there is no problem, and the number of steps is indicated in step S34 and the process is terminated. The number of machining times displayed here is the number of machining times until the forging die starts to be greatly worn, and is the number of forgings at which the life of the forging die does not expire at least up to this number of times.
On the other hand, if the wear becomes large before reaching the target number of times, the die life will not reach the target number of times as it is, so the forging process is corrected in step S38. Correct the shape.
5 and 12 show the correction results of the forging process. At first, since it was a process for forging through the shape shown in (I), as shown in FIG. Corresponding to having decided to forge through a shape, it becomes possible to forge with the damage form of a streak-like mark, and it turns out that it is corrected to a reasonable forging process, and the drastic improvement in forging life can be expected.
By doing in this way, it is corrected to the forging process or forged product shape which can obtain the target forging die life.
[0026]
In the above embodiment, the cumulative friction work is employed as the mechanical load, but various parameters shown in FIG. 14 can also be employed. FIG. 15 shows the surface area expansion ratio in one shot, and even if the surface area expansion ratio at the end of one shot is adopted, the damage form can be predicted accurately. In addition, it has been confirmed that a sliding distance or a sliding speed may be taken as the mechanical load.
[0027]
According to the present embodiment, it is possible to utilize the prediction results of the damage form and the die life in the forging process and the forging die preparation stage, and it is possible to rationally determine whether or not correction is necessary. Eventually, a reasonable forging preparation work becomes possible.
[Brief description of the drawings]
FIG. 1 is a flowchart showing an overall procedure performed in an embodiment of the present invention.
FIG. 2 shows a state in which a damage form is output by inputting a cumulative friction work-yield strength ratio in cumulative friction work-yield strength ratio-damage form D / B.
FIG. 3 shows “cumulative friction work-yield strength ratio-damage form” D / B obtained from a forging die used in the past.
FIG. 4 shows the form of damage that occurs in the forging die.
FIG. 5 shows a comparison of two forging processes.
FIG. 6 shows a heat-deformation coupled analysis of a workpiece during forging.
FIG. 7 shows how the cumulative friction work is accumulated during one forging process.
FIG. 8 shows a temperature change generated in a forging die during one forging process.
FIG. 9 shows how the hardness of a forging die decreases due to intermittent heating and cooling.
FIG. 10 illustrates the relationship between the forging die temperature and the yield strength.
FIG. 11 shows that the yield strength ratio changes with the number of forging processes.
FIG. 12 shows an example in which the form of damage that occurs is changed from a large wear form to a streak-like form by changing the process.
FIG. 13 shows various forms of damage that occurred in the forging die.
FIG. 14 illustrates various parameters that can be adopted as a mechanical load.
FIG. 15 shows how the surface area expansion ratio changes during one forging process.

Claims (4)

Any of the cumulative friction work, surface area expansion ratio, slip distance and slip speed is defined as mechanical load, and yield strength ratio is defined as thermal load, and mechanical load and thermal acting on the forging die being planned. Calculate the load and input the calculated load combination into the database for inputting the mechanical load and thermal load acting on the forging die and outputting the damage form generated in the forging die in the combined state of the load And predicting the damage form generated in the forging die by outputting the damage form from the database. Prediction method of claim 1, wherein the mechanical load is characterized cumulative frictional work amount der Rukoto. The forging die used in the past, claim 1, to a combination of mechanical load and thermal load, characterized in that to create the forging die before Symbol database to investigate the failure mode that occurs at the time Or the prediction method of 2. Any of the cumulative friction work, surface area expansion ratio, slip distance and slip speed is defined as mechanical load, and yield strength ratio is defined as thermal load, and mechanical load and thermal acting on the forging die being planned. The load is calculated for each forging process, and the combination of the load calculated for each forging process is generated in the forging mold by inputting the mechanical load and thermal load acting on the forging mold and combining the loads. A method for predicting the life of a forging die based on the number of forging processes that are input to a database for outputting a damage form and the damage form output from the database becomes a large wear form.

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